Numerous expansion processes are commonly used for hydrocarbon liquids recovery in the gas processing industry, and particularly in the recovery of ethane and propane from high pressure feed gas. Such expansion will provide at least in part for the refrigeration requirement in the hydrocarbon separation process. Additional propane refrigeration may be required where the feed gas pressure is low or where the feed gas contains significant quantity of propane and heavier components.
For example, the feed gas in most known NGL expander plants is cooled and partially condensed by heat exchange with demethanizer overhead vapor, side reboilers, and/or external propane refrigeration. The so formed liquid portion (containing less volatile components) is separated, while the vapor portion is typically split into two portions, with one portion being further chilled and fed to an upper section of the demethanizer while the other portion is typically letdown in pressure in a turbo-expander and fed to a mid section of the demethanizer. Such known configurations are commonly used for feed gas with relatively low CO2 (less than 2%) and relatively high C3+ (greater than 5%) content, and are generally not applicable for feed gas with high CO2 content (greater than 2%) and low C3+ content (less than 2% and typically less than 1%).
However, in many expander processes, the residue gas from the fractionation column still contains significant amounts of ethane and propane hydrocarbons that could be further recovered if chilled to an even lower temperature, or subjected to another rectification stage. Lower temperatures are typically accomplished using a higher expansion ratio across the turbo-expander to thereby lower the column pressure and temperature. Unfortunately, in most common configurations high ethane recovery in excess of 90% is neither achievable due to CO2 freezing in the demethanizer, nor economically justified due to the high capital cost of the compression equipment and energy costs. In other known plants, where configurations were adapted to relatively high propane and heavier recoveries, ethane recovery is typically in the 20% to 50% range.
Exemplary NGL recovery plants with a turbo-expander, feed gas chiller, separators, and a refluxed demethanizer are described, for example, in U.S. Pat. No. 4,854,955 to Campbell et al. Here, a configuration is employed for moderate ethane recovery with turbo-expansion in which the demethanizer column overhead vapor is cooled and condensed by an overhead exchanger using refrigeration generated from feed gas chilling. Such additional cooling step condenses most of the propane and heavier components from the column overhead gas, which is later recovered in a separator, and returned to the column as reflux. Unfortunately, while high propane recovery can be achieved with such processes, ethane recovery is frequently limited to 20% to 50% due to CO2 freezing problems in the demethanizer when processing a high CO2 feed gas.
Most known plants typically require very low temperatures (e.g., −100° F. or lower) in the demethanizer in order to achieve a high ethane recovery. However, as in many high propane recovery configurations, the CO2 content in the top trays will increase due to the very low temperatures, which invariably causes significant internal recycle and accumulation of CO2. Thus, such configurations typically result in high CO2 concentrations in the top trays, and are thus more prone to CO2 freezing, which presents a significant obstacle for continuous operation. Alternatively, CO2 concentration can be reduced in the feed gas to a tolerable limit with the use of amine CO2 removal units. However, such CO2 removal option adds significant cost and energy consumption to the plants.
To circumvent the CO2 freezing problems in the demethanizer of an NGL plant, CO2 can be removed in the NGL fractionation column. For example, U.S. Pat. No. 6,182,469 to Campell et al. discloses a configuration in which a portion of the liquid in the top trays of the demethanizer is withdrawn, heated, and returned to the lower section of the column for CO2 removal and control. While such configuration can remove undesirable CO2 at least to some degree, the fractionation efficiency of the demethanizer is often reduced and additional fractionation trays, heating, and cooling duties must be provided for such processing. Yet another approach for processing feed gas with concurrent CO2 removal is described in U.S. Pat. No. 6,516,631 to Trebble in which deethanizer overhead vapor is recycled to the mid section of the demethanizer for removal of CO2. Such recycle schemes can also be used to reduce CO2 content in the NGL product to at least some degree, but deethanizer vapor recycling requires additional compression, heating, and cooling that often make such configurations economically less attractive.
Thus, numerous attempts have been made to improve the efficiency and economy of processes for separating and recovering ethane and heavier natural gas liquids from natural gas and other sources. However, all or almost all of them are relatively complex and often fail to achieve economic operation for high ethane recovery with high CO2 feed gases. Consequently, there is still a need to provide improved methods and configurations for natural gas liquids recovery.